Method for producing polyurethane elastomer fibers and fibers produced according to this method

ABSTRACT

Polyurethane elastomer fibers with superior mechanical and heat distortion properties are obtained by a method in which 
     (a) a segmented polyurethane polymer is produced on the basis of a macro-diol, an aliphatic diisocyanate, and a chain extender with at least two hydroxy and/or amino groups, where the polymer has a molar excess of isocyanate groups over the hydroxy and/or amino groups from the macro-diol and chain extender; 
     (b) the polyurethane polymer is melt-extruded to form a fiber; and 
     (c) The extruded fiber is subjected to a post-treatment. 
     Steps (a) and (b) are carried out under temperature conditions and within a time interval where essentially no allophanate will be formed, while step (c) is performed under temperature conditions and within a time interval in which the polyurethane polymer is cross-linked through the formation of allophanate.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing polyurethane elastomerfibers, and it also relates to the fibers that are produced inaccordance with the inventive method.

Polyurethane elastomers are block polymers with a regular structure ofsoft and hard segments. The soft segments consist of long, flexiblechains lacking any order of arrangement, which give the requiredrubber-like elasticity to the fiber. The properties of the fiber inregard to stretching and tensile strength can be varied depending on themolar mass and the kind of soft segment. The hard segments serve tofixate the soft segments. After a deformation, the return of themolecular chains of the soft segments to their former length occurs inan entropy-elastic manner. The hard segments consist of partiallycrystalline domains with a structure of short chains. The main task ofthe hard segments is to serve as anchor points to prevent a slippage ofthe polymer chains under the influence of mechanical forces. Followingan elongation, the shape-restoring forces present in the elastomer willcause a contraction to nearly the original length. The remainingdifference in length is called permanent elongation.

In general, polyurethane elastomers are produced in a single step or ina two-step process. In the first step of the two-step process,pre-polymers are formed in a reaction between diols of a highermolecular order and diisocyanates. In a second step, high-molecular endproducts are formed as the pre-polymers react with so-called chainextenders. Excess quantities of diisocyanate are used in the reaction ofthe first step, so that the pre-polymer molecule is terminated byisocyanate groups at both ends. The chain extenders are bi-functional,low-molecular compounds terminated by hydrogen atoms, mostly dihydroxyand diamino compounds capable of entering into reactions. The dihydroxyand diamino compounds react with the pre-polymers in the formation ofthe corresponding carbamic acid derivatives, i.e., the polyurethaneelastomers, specifically poly-urea urethane elastomers. In themacro-molecular chains, the soft segments of higher-molecular diolsalternate with the rigid hard segments resulting from the reaction ofthe chain extenders terminated by iso-cyanate groups. The single-stepprocess (also called one-shot process) bypasses the pre-polymer stage,as the diisocyanate reacts simultaneously with the macro-diol and thechain extender.

The difference in the chemical composition of hard and soft segments aswell as the difference in their polarities and molecular weights willcause the hard and soft segments to segregate from each other. Theformation of hydrogen bridges between neighboring chains has the effectthat the hard segments congregate in parallel alignment with each other.The long movable molecule chains in between develop cross-linkedinterconnections that are loosened and stretched when the loose-knitnetwork is subjected to a tensile deformation. The interaction betweenthe hard segments prevents plastic flow of the molecular chains in thedistended state. The stretching of the macro-molecules is associatedwith a transition into a more highly ordered state and a decrease inentropy. When the mechanical stress load is removed, the thermal motionof the molecules will cause them to return to the cross-linked statethat corresponds to a higher level of entropy. Strong mechanical stressloads, however, will break the interaction between the hard segments,causing irreversible structural rearrangements of the hard segments.This has a negative effect on the mechanical hysteresis properties.Especially melt-extruded polyurethane fibers exhibit a large hysteresisloss of elastic energy and force as well as a high amount of permanentelongation. In order to improve the properties of the fibers, it willtherefore be necessary to provide a better fixation of the hardsegments.

The known methods of producing melt-extruded polyurethane elastomerfibers primarily use a polyurethane polymer based on aromaticdiisocyanates, mostly di-phenyl methane-4,4′-diisocyanate (MDI). Thepolymer coming out of the reaction is melted and processed into a fiberby way of a melt-extrusion process. However, polyurethane polymers basedon aromatic diisocyanates are becoming less acceptable, because theirdecomposition releases aromatic amino groups that are suspected of beingcarcinogens. In addition, polyurethane polymers based on aromaticdiisocyanates have a tendency of yellowing.

OBJECT OF THE INVENTION

It is therefore the object of the present invention to provide a methodof producing polyurethane elastomer fibers based on non-aromaticdiisocyanates with improved properties, particularly with respect totear strength, tear elongation, permanent elongation and heat distortiontemperature (HDT).

SUMMARY OF THE INVENTION

In accordance with the present invention, the objective is met by amethod that has the following steps:

(a) A segmented polyurethane polymer is produced on the basis of i) amacrodiol with a molecular weight of approximately 500 to 10,000, ii) analiphatic, cyclo-aliphatic and/or aliphatic-cycloaliphatic diisocyanateand iii) a chain extender with at least two hydroxy and/or amino groups.As a percentage of the sum of the hydroxy and amino groups, the polymerhas a molar excess of isocyanate groups of at least approximately 0.2%over the hydroxy and/or amino groups from the macrodiol and chainextender.

(b) The polyurethane polymer is melt-extruded to form a fiber. Steps (a)and (b) are carried out under temperature conditions and within a timeinterval where essentially no allophanate will be formed.

(c) The fiber is subjected to a post-treatment under temperatureconditions and within a time interval in which the polyurethane polymeris cross-linked through the formation of allophanate.

The polyurethane polymer must melt and be in the liquid phase at asuitable temperature. The polyurethane polymer is produced under thepre-polymer or the one-shot method, either through a reaction betweenmacro-diol, chain extender and diisocyanate with the optional additionof a catalyst, essentially without a solvent, or by melting apreliminary form of polyurethane polymer containing iso-cyanate groupsin stoichiometric proportion or in deficit proportion to hydroxy- andamino groups and by allowing the melted material, possibly after it hascooled down, to react with a diisocyanate and/or anisocyanate-terminated pre-polymer, again essentially in the absence of asolvent.

In preferred embodiments, the polyurethane polymer has a molar excess ofiso-cyanate groups in relation to hydroxy- and amino groups of about0.2% to 15%, the range from 1% to 10% being especially preferred.Polyurethane-polymer chains can be cross-linked through the formation ofallophanate- or biuret bonds (Subsequently, only the term “allophanate”will be used. Depending on the context, this is meant to include“biuret”). In this case, an excess iso-cyanate group reacts with analready formed urethane or urea group, causing a branch in the molecularstructure. The applicant has made the observation thatallophanate-cross-linked polyurethane polymers based on aliphaticdiisocyanates are unsatisfactory for the melt-extrusion process. Theextrusion temperatures required for allophanate-cross-linkedpolyurethanes are in the area of 230° C. The extruded fibers have a highdegree of stickiness and inadequate strength. At the high extrusiontemperature required, there is furthermore a strong decay in the molarmass of the polymer. The inventive method makes use of the difference inthe reaction kinetics between the polyurethane chain formation and theallophanate formation. The formation of the allophanate links occursmore slowly than the build-up of the linear polyurethane chains.Consequently, the polyurethane polymer is produced with a defined excessproportion of isocyanate and is extruded even before the formation ofallophanate cross-links sets in. The not yet cross-linked polyurethanecan be extruded at relatively low temperatures, so that a thermaldecomposition of the polymer is avoided and the tendency of the fiberstowards stickiness is reduced. The further treatment of the fibers(winding on spools, tempering, etc.) presents no problems. A subsequentformation of covalent cross-links in the hard segments takes place inthe post-treatment of the fibers. In addition to the physicalcross-linking by way of hydrogen bridges, a chemical network ofallophanate bonds is built up. In comparison to conventionally producedmelt-extruded Elastane™ fibers, the inventive method brings a clearimprovement of the fiber properties. As an essential advantage, thealiphatic allophanate bonds that are present in the fibers producedaccording to the invention have significantly better thermal stabilitythan aromatic allophanate bonds.

A person skilled in the art will be able to find, through simpleexperimentation, suitable temperatures and time intervals for producingand melt-extruding the polyurethane polymer without a significant amountof allophanate being formed in the process. The formation of allophanatebecomes noticeable by the fact that the polymer is no longer completelysoluble in customary polyurethane solvents such as, e.g., dimethylformamide (DMF) or dimethyl acetamide (DMA). The following informationwill serve as a general frame of reference: The polymer can be kept forabout two hours at a temperature of 150° C. without an appreciableamount of allophanate formation taking place. If the temperature islowered by 10° C., the time frame is extended by a factor of 1.2, sothat at a temperature of 80° C., about 7 hours will be available. Thefollowing formula represents an approximation of the relationshipbetween the length of time and the temperature.

t ₁≦2h×1.2 exp[(425K−T ₁)/(10K)]

It is practical to perform the extrusion on conventional equipment withfiber strengths of 5 to 2000 dtex. The preferred temperature for themelt extrusion is about 80° C. to 180° C., with particular preferencefor the range from 100° C. to 150° C.

The post-treatment can be a heat-curing process of several hours at apreferred temperature range of 60° C. to 100° C. or, alternatively,room-temperature curing for several days. A post-treatment attemperatures higher than 150° C. is not recommended. The extruded fiberscan be laid out on a conveyor belt and sent through a conveyor oven.They can also be wound on spools or bobbins and placed in an oven orenvironmental chamber. The following formula will serve as a referencefor determining the required length of time as a function of thepost-treatment temperature:

t ₂≦5h×1.2 exp[(425K−T ₂)/(10K)]

With respect to the macro-diols, the preference is for essentiallylinear diols which, except for the hydroxyl groups at the ends, carry noother groups that react with isocyanates. The macro-diols have amolecular weight of about 500 to 10,000, preferably 700 to 5000, withspecial preference for the range from 1000 to 3000. The molecular weightis meant as a weight-averaged mean molecular weight. If the macro-diolrest molecules become too short, the difference in cohesion energybetween hard and soft segments becomes greater, causing a greater amountof mixing between the phases and thus inferior elastic properties.Macro-diols with a low glass transition point are preferred. In general,the macro-diols used have glass transition temperatures of about −35° C.to −60° C.

The preference is for using polyester- or polyether glycols. The termpolyether glycols means hydroxyl-group terminated polyethers.Polyalkylene glycols are preferred, as for example polyethylene glycol,polypropylene glycol and/or poly-tetra-methylene glycol, with specialpreference for the last among the foregoing examples.Poly-tetra-methylene glycol, also known as poly-tetra-hydrofuran, can beproduced by ionic polymerization of tetra-hydrofuran with acidcatalysts. Suitable copolymers are also obtained by polymerizingtetra-hydrofuran with a mixture of propylene oxide, ethylene oxide andglycols. Elastomers that are synthesized from polyether glycols aredistinguished by advantageous properties at low temperatures and a highdegree of hydrolytic stability.

Suitable polyester glycols are produced preferably through anester-forming reaction of an aliphatic and/or cyclo-aliphatic dicarbonicacid with excess quantities of a diol.

Among the preferred kinds of dicarbonic acids are succinic acid,glutaric acid, adipic acid, pimelic acid, azelaic acid and sebacic acid.The dicarbonic acid is converted into an ester with an excess quantityof diol, preferably ethylene glycol, diethylene glycol, triethyleneglycol, propylene glycol, 1,4-butane diol, 1,5-pentane diol and/or1,6-hexane diol. Particularly preferred is a polyester of adipic acidand ethylene glycol. Polyester segments have a tendency to crystallizeat low temperatures, at the expense of the elastic properties. Thecrystallizing tendency of polyester chains can be reduced preferably bythe incorporation of methyl branches.

This can be accomplished by a partial replacement of the aforementioneddiols by other diols such as 1,2-propane diol and 2,3-butane diol, or byusing methyl-substituted dicarbonic acids. By using the aforementionedlonger-chained glycols, such as 1,4-butane diol, 1,5-pentane diol and/or1,6-hexane diol, elastomers of increased hydrolytic stability areobtained.

Suitable polyester glycols can also be obtained by a reaction ofomega-hydroxy carbonic acids with small amounts of diols or by aring-opening polymerization of lactones with small quantities of diol.It is also possible to use mixtures of polyether glycols and polyesterglycols. As a reference for suitable macro-diols, see also Ullmann'sEncyclopedia of Technical Chemistry, 3d edition 1963, published by Urban& Schwarzenberg, Munich and Berlin, vol. 14, pp. 344 f.

The aliphatic, cycloaliphatic and/or aliphatic-cycloaliphaticdiisocyanate preferably includes (beyond the isocyanate groups) analkylene group with 2 to 14 carbon atoms, a cyclo-alkylene group with 5to 8 carbon atoms and/or an aliphatic-cycloaliphatic group with 7 to 24carbon atoms. Particularly preferred are hexamethylene diisocyanateand/or dicyclohexyl methane-4,4′-diisocyanate.

The chain extender is a compound containing at least two hydroxy- and/orprimary amino groups, preferably a diol or a diamine, which have a lowmolecular weight in comparison to the macro-diol. In particular, thecompounds used for chain extenders are diols, diamines or amino alcoholswith 2 to 6 carbon atoms. Preferred are, e.g., ethylene glycol,1,4-butane diol, cis-2-butene-1,4-diol and 2-butyne-1,4-diol.

Olefin-unsaturated chain extenders are used in one embodiment. The term“olefin-unsaturated” is meant to indicate that the chain extender hasone or several double or triple bonds capable of a polymerizationreaction. The olefin-unsaturated chain extender can be a diamino alkene,diamino-alkine, diamino cycloalkene, alkene diol, alkine diol and/orcycloalkene diol. Examples of suitable diamines are cis- ortrans-1,4-diamino-but-2-ene, cis- or trans-4,4′-diamino stilbene,diamino maleic acid dinitrile, 1,4-diamino but-2-yne and/or 3,6-diaminocyclohexene-(1). Preferred examples of suitable diols areglycerin-l-allyl ether, cis- or trans-2-butene-1,4-diol,2-butyne-1,4-diol and 5,6-bis-(hydroxy methyl)-bicyclo[2.2.1.]heptene-2.The use of olefin-unsaturated chain extenders permits a furtherimprovement of the mechanical textile properties of the fibers made ofthe polyurethane elastomers according to the invention in that itinduces the covalent cross-linking of the double and triple bonds thatare built into the polymer chains. This is accomplished by exposing theextruded fibers to high-energy radiation. Preferably, the fibers aretreated with electron beams or UV radiation.

Polyurethane elastomers according to the invention can contain additivessuch as matte-finish agents, pigments, antioxidants, thermalstabilizers, photo- and specifically UV stabilizers and/or hydrolysisstabilizers.

The polyurethane polymer with free isocyanate groups does not have astable shelf life because of the allophanate cross-linking that wouldoccur during the storage. It is therefore produced immediately prior tothe extrusion process. Under the first of two available methods, thepolymer is produced directly from the components according to either thesingle-step process or the pre-polymer process. The macro-diol, thechain extender and the diisocyanate are brought together in the requiredquantities at about 60 to 180° C., preferably between 80 and 150° C. forthe melt reaction. Optionally, a poly-addition catalyst may be added,preferably dibutyl tin dilaurate or dibutyl tin diacetate, to set adesired reaction level. When using the pre-polymer method, macro-dioland diisocyanate are first combined into the pre-polymer, which issubsequently extended by the chain extender to form the desiredpolyurethane polymer. The resulting polymer material is immediatelyextruded into fibers.

Alternatively, i.e., under the second of the two methods, a stablepreliminary form of polyurethane polymer is produced in a first phase,containing isocyanate groups in stoichiometric proportion or in deficitproportion to hydroxy-and amino groups. The material can be made into agranulate and put into intermediate storage if necessary. To arrive atthe final form of polyurethane to be extruded into fibers, thepreliminary product is melted, if necessary. At a preferred temperatureof about 100 to 160° C., an aliphatic, cycloaliphatic and/oraliphatic-cycloaliphatic diisocyanate and/or an isocyanate-terminatedpre-polymer is added, and the mixture is homogenized. Possibly, thepreliminary polymer product could be melted at a higher temperature thanthe preferred reaction temperature for the mixture, in which case it isgood practice to let the melted preliminary product cool down to theappropriate level before adding the diisocyanate and/orisocyanate-terminated pre-polymer. Particularly suitable asisocyanate-terminated pre-polymers are reaction products of macro-diolwith 1.1 to 3 molar equivalents of diisocyanate. The preliminary polymerproduct can be melted in an extruder, in which case the diisocyanateand/or isocyanate-terminated pre-polymer is added preferably eitherclose to the extruder orifice or after the extruder in the melt-materialconduit. It is recommended practice to homogenize the mixture, e.g.,with static mixers in the melt-material conduit prior to the extrusionof the fibers.

The preferred molar ratio between macro-diol and chain extender inpolyurethane polymer is between about 1:4 and 1:1.

The melt-extruded polyurethane fibers made according to the inventivemethod have clearly superior properties in comparison to conventionalmelt-extruded polyurethane fibers. The covalent cross-linking of thehard segments through the allophanate bonds significantly improves thehysteresis properties, i.e., the permanent elongation and the loss ofelastic tension are significantly reduced while the tear strength andheat distortion temperature are increased.

The invention will be discussed in closer detail based on the followingexamples.

EXAMPLE 1

A quantity of 100 g (0.05 mol) of polytetrahydrofuran (PTHF, molarweight 2000 g/mol; OH-number 57.3) was measured out into a Teflon™vessel and heated to 100° C. In 10-minute intervals and with vigorousstirring, 8.99 g (0.102 mol) of butene diol, 28.3 g (0.168 mol) ofhexamethylene diisocyanate (HDI), as well as 3 mg of dibutyl tindiacetate as a catalyst, were added. About five minutes after adding thecatalyst, the viscosity of the reacting mixture increases stronglybecause of the build-up in molecular weight. The stirring speed wasreduced at this point in order to assure a uniform mixing of the highlyviscous polymer melt. To complete the reaction, stirring was continuedfor another 20 minutes at a temperature of 100° C.

The polyurethane melt, which still contains free isocyanate groupscorresponding to the excess of HDI, was used immediately for themelt-extrusion process in a piston extruder. The extrusion temperaturewas 80° C. and the holding time was 30 minutes. The fiber produced bythis process was not sticky and could be wound onto spools without aproblem. After a storage interval of two days at room temperature, thefiber was heat-cured for 24 hours at 100° C. The polyurethane elastomerfiber produced in accordance with this example was no longer soluble indimethylacetamide (DMA) or dimethylformamide (DMF), which indicates thepresence of allophanate cross-linking. The fiber had significantlyimproved properties in comparison to a conventionally melt-extrudedfiber (see table).

EXAMPLE 2 Reference for Comparison

The process of example 1 was repeated, but the quantity of HDI wasreduced to 25.6 g (0.152 mol), i.e., a stoichiometrically matchedquantity with no excess in relation to PTHF and butene diol.

The fibers produced according to examples 1 and 2 were subjected to atest. Measurements of force vs. elongation were made on a Zwick Model1435 tensile tester. All measurements were made at standardenvironmental conditions. The measurement methods were based on DIN53835. The following instrument parameters were selected for themeasurement of tear strength and tear elongation: clamping length 50 mm;pre-tensioning force 0 N; elongation rate 500 mm/minute. DIN 53835 part2 was used for guidance in measuring the permanent elongation. Thefibers were stretched and returned to the unstretched condition fivetimes in a constant elongation interval. The instrument recorded thefirst and fifth of the stretching and unstretching cycles. From theplotted diagram, the permanent elongations and the mechanical parameterb_(w,5) were obtained. The permanent elongation given in the tablerepresents the ratio between the remaining increase in length and theoriginal length of the test specimen. The dimensionless parameterb_(w,5) represents the relative loss in elastic tensioning force fromthe first to the fifth stretching cycle. The following instrumentparameters were used in the measurement of the permanent elongation andthe loss of elastic tensioning force: clamping length 100 mm, elongation300%, pre-tensioning force 0.01 cN/tex, elongation rate 500 mm/minute,number of stretch cycles 5. The heat-distortion temperature (HDT) wasmeasured with a Perkin-Elmer TMA 7 thermo-mechanical analyzer at thefollowing settings: static force 0.002 cN/dtex; rate of temperatureincrease 2 K/minute. The results are summarized in Table 1.

TABLE 1 Example 1 2 3 Molar ratio PTHT/butene diol/HDI 1/2/3.3 1/2/31/2/3.3 η_(rel) (0.5 wt. % solution in DMA) insoluble 1.34 insolublePerm. elongation, 1st cycle [%] 35 95 30 Perm. elongation, 5th cycle [%]45 115 41 Tear strength [cN/tex] 7.0 3.4 3.5 Tear elongation [%] 700 800600 Loss in tensioning force b_(w,5) 0.23 0.24 0.10 HDT [° C.] 125 90175

EXAMPLE 3

The process of example 1 was repeated, but prior to extruding, thepolymer melt was heat-cured for 20 hours at 80° C., in order to formallophanate cross-linking bonds in the polymer. The extrusiontemperature needed for extruding the allophanate-linked polyurethane was230° C. (holding time about one hour). The extrudability wasunsatisfactory. The fibers resulting from the process had a high degreeof stickiness and low tear strength. It was impossible to wind thefibers on a spool. Subsequent heat-curing of individual fibers made nosignificant improvement.

EXAMPLE 4

The fiber produced in the process of Example 1 was irradiated with aDurr electron-beam hardening apparatus with a radiation dose of 200 kGy.The results are given in Table 1 above, demonstrating that a furthercross-linking of the hard segments yields an additional degree ofimprovement.

What is claimed is:
 1. A method for producing polyurethane elastomerfibers, comprising the steps of (a) producing a segmented polyurethanepolymer based on i) a macro-diol with a molecular weight ofapproximately 500 to 10,000, ii) at least one diisocyanate belonging tothe class consisting of aliphatic, cyclo-aliphatic andaliphatic-cycloaliphatic diisocyanates, and iii) a chain extender withat least two molecular groups belonging to the class consisting ofhydroxy and amino groups, the polymer having a molar excess ofisocyanate groups of at least approximately 0.2% over the combined totalof hydroxy and amino groups from the macro-diol and chain extender, saidpercentage of 0.2% being in relation to the combined total of hydroxyand amino groups, wherein macro-diol, chain extender and diisocyanate,with the optional addition of a catalyst, are brought together to reactwith each other, essentially in the absence of a solvent; (b)melt-extruding the polyurethane polymer to form a fiber, wherein steps(a) and (b) are carried out under temperature conditions and within atime interval where essentially no allophanate will be formed; (c)subjecting the fiber to a post-treatment under temperature conditionsand within a time interval in which cross-linking of the polyurethanepolymer will occur through the formation of allophanate; and (d)exposing the extruded fiber to high-energy radiation to effect an atleast partial cross-linking of the polyurethane elastomer.
 2. A methodfor producing polyurethane elastomer fibers, comprising the steps of (a)producing a segmented polyurethane polymer based on i) a macro-diol witha molecular weight of approximately 500 to 10,000, ii) at least onediisocyanate belonging to the class consisting of aliphatic,cyclo-aliphatic and aliphatic-cycloaliphatic diisocyanates, and iii) achain extender with at least two molecular groups belonging to the classconsisting of hydroxy and amino groups, the polymer having a molarexcess of isocyanate groups in the order of 0.2% over the combined totalof hydroxy and amino groups from the macro-diol and chain extender, saidpercentage of 0.2% being in relation to the combined total of hydroxyand amino groups, wherein a preliminary form of polyurethane polymercontaining iso-cyanate groups in a proportion to hydroxy- and aminogroups not exceeding the stoichiometric ratio is brought to a meltedcondition and, after an optional cooling-down time interval, is broughttogether into reaction with at least one of the reagents from the classconsisting of diisocyanate and isocyanate-terminated pre-polymers,essentially in the absence of a solvent; (b) melt-extruding thepolyurethane polymer to form a fiber, wherein steps (a) and (b) arecarried out under temperature conditions and within a time intervalwhere essentially no allophanate will be formed; (c) subjecting thefiber to a post-treatment under temperature conditions and within a timeinterval in which cross-linking of the polyurethane polymer will occurthrough the formation of allophanate; and (d) exposing the extrudedfiber to high-energy radiation to effect an at least partialcross-linking of the polyurethane elastomer.
 3. The method according toclaim 1, wherein the polymer has a molar excess of isocyanate groupsessentially in the range from 0.5% to 15% over the combined total ofhydroxy and amino groups.
 4. The method according to claim 1, whereinthe macro-diol has a molecular weight essentially in the range of 1000to
 3000. 5. The method according to claim 1, wherein the macro-diolcomprises at least one of the class consisting of polyether glycols andpolyester glycols.
 6. The method according to claim 5, wherein thepolyether glycol is a polyalkylene glycol.
 7. The method according toclaim 6, wherein the polyalkylene glycol comprises at least one of theclass consisting of polyethylene glycols, polypropylene glycols andpolytetramethylene glycols.
 8. The method according to claim 5, whereinthe polyester glycol comprises the polyester of a diol and at least oneof the class consisting of aliphatic and cycloaliphatic dicarbonicacids.
 9. The method according to claim 8, wherein the dicarbonic acidcomprises at least one of the class consisting of succinic acid,glutaric acid, adipic acid, pimelic acid, azelaic acid and sebacic acid.10. The method according to claim 8, wherein the diol comprises at leastone of the class consisting of ethylene glycol, diethylene glycol,triethylene glycol, propylene glycol, 1,4-butane diol, 1,5-pentane dioland 1,6-hexane diol.
 11. The method according to claim 1, wherein thediisocyanate comprises at least one of the class consisting of analkylene group with 2 to 14 carbon atoms, a cyclo-alkylene group with 5to 8 carbon atoms, and an aliphatic-cycloaliphatic group with 7 to 24carbon atoms.
 12. The method according to claim 11, wherein thediisocyanate comprises at least one of the class consisting ofhexamethylene diisocyanate and dicyclohexyl methane-4,4′-diisocyanate.13. The method according to claim 1, wherein the chain extendercomprises at least one of the class consisting of diamines, diols andamino alcohols with two to six carbon atoms.
 14. The method according toclaim 1, wherein the chain extender is olefin-unsaturated.
 15. Themethod according to claim 14, wherein the chain extender comprises atleast one of the class consisting of cis-1,4-diamino-but-2-ene,trans-1,4-diamino-but-2-ene, cis-4,4′-diamino stilbene,trans-4,4′-diamino stilbene, diamino maleic acid dinitrile, 1,4-diaminobut-2-yne, 3,6-diamino cyclohexene-(1), cis-1,4-but-2-ene diol,trans-1,4-but-2-ene diol, 1,4-but-2-yne diol, and 5,6-bis-(hydroxymethyl)-bicyclo[2.2.1.]heptene-2.
 16. The method according to claim 1,wherein the step of melt-extruding the polyurethane polymer is carriedout at a temperature essentially within the range of 80° C. to 180° C.17. The method according to claim 1, wherein the steps (a) and (b) arecarried out at a temperature T₁ and within a time interval t₁ meetingthe condition that2h ≥ ∫_(t = 0)^(t = t₁)1.2 × exp [(T₁ − 425  K)/10  K]t.


18. The method according to claim 1, wherein the post-treatment iscarried out at a temperature T₂ and within a time interval t₂ meetingthe condition that5h ≤ ∫_(t = 0)^(t = t₂)1.2 × exp [(T₂ − 425  K)/10  K]t.


19. The method according to claim 1, wherein the molar ratio betweenmacro-diol and chain extender in the polyurethane polymer is essentiallyin the range between 1:4 and 1:1.
 20. The method according to claim 1,wherein the high-energy radiation is one of the class consisting ofelectron-beam radiation and UV radiation.